Electromagnetic coal seam gas recovery system

ABSTRACT

A system for recovering gas trapped within the earth includes a casing ( 24 ) sized and configured to be positioned within a borehole in the earth, the casing ( 24 ) formed of a material that is transmissive to electromagnetic energy and gas within the earth; an antenna ( 40 ) sized and configured to be positioned within the casing ( 24 ). The antenna ( 40 ) has a distal end and a proximal end and including a radiating element at the distal end of the antenna ( 40 ) which, in operation, transmits electromagnetic energy toward a desired area of the earth, and an interior channel for allowing gas to be conveyed from the distal end to the proximal end of the antenna ( 40 ).

This application claims the benefit of Provisional Application No.60/256,367, filed Dec. 18, 2001.

BACKGROUND

The invention relates to the recovery of gas from subterraneanformations in the earth.

Extensive and high volumes of hydrocarbon gases (e.g., methane) trappedwithin coal seams have been discovered in various parts of the UnitedStates. For example, large amounts of trapped methane gas have beendiscovered in eastern Wyoming (see, for example, “Powder River BasinCoalbed Methane Play Heats Up,” E&P Perspectives, Vol. X, R57, Oct. 22,1998 (attached herewith). Naturally occurring degradation processes,such as the biodegradation of microorganisms in the coal is believed tocause the generation of the methane gas trapped within the coal seams.

Methods of economic and environmentally sound gas recovery are underway.A major problem encountered is the large amount of aquifers (water) thatimpedes the ability to recover the gas from bore holes drilled in to thecoal seam. Specifically, the in-ground water serves as a barrier to theeffective removal of the gas from the bore hole. The water must beremoved by a pump or redirected to allow more efficient removal of thegas. Systems of co-generation of power for pumps are being consideredfor the prime supply of electrical energy for the pumps. That is, theelectrical power for operating gas turbines used to drive the pumpscould be generated using a portion of the gas removed from the borehole.

SUMMARY

In a general aspect of the invention, a system for recovering gastrapped within the earth, the system includes a casing sized andconfigured to be positioned within a borehole in the earth, the casingformed of a material that is transmissive to electromagnetic energy andgas within the earth, and an antenna sized and configured to bepositioned within the casing. The antenna includes a radiating elementat a distal end of the antenna which, in operation, transmitselectromagnetic energy toward a desired area of the earth, and aninterior channel for allowing gas to be conveyed from the distal end toa proximal end of the antenna.

In another aspect of the invention, a method for recovering gas trappedwithin the earth includes the following steps. A casing is positionedwithin a borehole in the earth, the casing formed of a material that istransmissive to electromagnetic energy and gas within the earth. Anantenna is positioned within the casing, the antenna having a distal endand a proximal end. The antenna includes a radiating element at thedistal end of the antenna which, in operation, transmits electromagneticenergy toward a desired area of the earth; and an interior channel forallowing gas to be conveyed from the distal end to the proximal end ofthe antenna. The method further includes applying electromagnetic energyto the antenna to radiate the earth surrounding the casing; drawing gaswithin the earth into the interior channel of the antenna at the distalend of the antenna; and conveying the gas within the interior channel tothe proximal end of the antenna.

Embodiments of these aspects of the invention may include one or more ofthe following features.

A product return pipe has a first end connected to the proximal end ofthe antenna and a removable cap attached to a second end of the productreturn pipe. A bellows is connected to the proximal end of the antenna.A thermocouple assembly is connected to the proximal end of the antenna.

The antenna is configured to operate in a frequency range between 300KHz and 300 GHz. More particularly, the frequency range is between 1 MHzand 100 MHz (e.g., about 27 MHz). The antenna is configured to operateat a power level in a range between 3 Kwatts and 20 Kwatts (e.g., about10 Kwatts).

Among other advantages, the system and method (1) reduce the negativeimpact of water on the in situ recovery of coal gas, such as methanefrom underground beds or seams of coal; and (2) provide additional orenhanced stimulation of gas production from the coal deposits.

The basic energy source proposed for reducing the water barrier effectand stimulating production in-situ is electromagnetics. Electromagneticenergy at frequencies as low as 60 Hz and extending into the microwavefrequencies supplied by earth electrodes in the form of antennas and/orwaveguides may be employed in the proposed processes. The basic idea isto introduce current into the subterranean formation to vaporize or boilthe water in a specified region of the coal seam. The currents arederived from the electromagnetic field energy absorbed by the coalmaterial and water.

Specific in-ground applicator structures such as rod electrodes,antennas or waveguides and transmission lines provide the inducedcurrents in the coal seam to vaporize a given amount of water. Forexample, antennas in a vertical or horizontal bore hole drilled in acoal seam radiate electromagnetic energy away from the antenna into thecoal creating a dry region around the bore hole/antenna structure. Apump can be used in conjunction with the antenna for water removal orthe bore hole containing the antenna may be pressurized to keep thewater away from the antenna/bore hole.

A special gas filtering system can be employed around the antenna(within or outside the bore hole) to permit gas recovery up to theantenna bore hole without water. This special filter would block liquidwater and allow only gas to pass through it.

The dry region around the antenna borehole created by dielectric heatingof the coal/water matrix is maintained by the power supplied by theantenna (e.g., 3 to 20 kilowatts on average). This dry region,maintained by either resistive (low frequency) currents or dielectric(high frequency) currents in the coal seam, allows the gas to betransferred from regions outside the casing to within the antenna case,bore hole, or adjacent recovery wells equipment with special filters andflow lines for ease of gas recovery without water.

The dry sheath region or zone is maintained at approximately 100° C. toensure that there is no liquid water.

Thermal energy is not a requirement for the gas deposits in place. As aresult of the dielectric sheath created by electromagnetic currents, theradiation fields of the antenna now extend further into the coal seamaway from the antenna bore hole thereby creating an enhanced zone orregion of heating and results in an enlargement of the dry zone and lessimpedance of gas flow to the recovery well by water.

Another benefit of electromagnetic heating is the enlargement offracture zones in the coal seams by steam pressure and thermalgradients. The result is enhanced flow of methane gas to recovery wells.

Still another benefit of electromagnetic heating is the increasedactivity of microorganisms from the thermal energy deposit, especiallyat radio frequencies.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the upper portion of an RF gas recovery system inaccordance with the invention.

FIG. 2 illustrates the lower portion of the RF gas recovery system ofFIG. 1.

FIG. 3 illustrates an alternative embodiment of a lower portion of theRF gas recovery system of FIG. 1.

FIG. 4 illustrates another alternative embodiment of the lower portionof the RF gas recovery system of FIG. 1.

FIG. 5 illustrates still another alternative embodiment of the lowerportion of the RF gas recovery system of FIG. 1.

FIG. 6 illustrates still another alternative embodiment of the lowerportion of the RF gas recovery system of FIG. 1.

FIG. 7 illustrates still another alternative embodiment of the lowerportion of the RF gas recovery system of FIG. 1.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, the upper portion of an RF gas recoverysystem 10 is shown for radiating electromagnetic energy into a coal seamdeposited with the ground 12 and extracting gas released by the heatinggenerated by the electromagnetic energy. In particular, gas recoverysystem 10 includes an outer casing 14 disposed within a borehole 16drilled deep within the ground. The outer casing 14 houses a coaxial RFapplicator 18 that includes a coaxial transmission line 20 extendingfrom the upper end of the antenna at the surface of the earth to adistal end of the antenna. The coaxial transmission line 20 includes acenter conductor 22 positioned coaxially within an outer conductor 24.In this embodiment, center conductor 22 and outer conductor 24 havediameters of about 1 inch and 2.9 inches, respectively, and have lengthsgreater than 30 feet. In general, the length of the RF applicator 18 andthe outer casing 14 can be between 8 and 200 feet. Insulative spacers(e.g., Teflon) 26 are spaced along the length of the center conducter 22to maintain its coaxial position relative to the outer conductor 24.Furthermore, due to the relative long length of RF applicator 18,support collars 27 are spaced periodically along the length of outerconductor 24. The upper end of the coaxial transmission line 20 isconnected to an RF generator (not shown) via an RF coax line 30. Theupper ends of center conductor 22 and outer conductor 24 of coaxialtransmission line 20 include expansion joints in the form of bellows 31and 32, respectively.

As shown in FIG. 2, in this embodiment, the distal end of the RFapplicator includes a dipole antenna 40 extending between 5–6 feet fromthe end of coaxial transmission line 20. Dipole antenna 40 has adiameter larger than coaxial transmission line 20. A collar 41 isattached at the transition between dipole antenna 40 and coaxialtransmission line 20 to provide mechanical support and to ensure agas-tight seal between outer conductor 24 of transmission line 20 andouter conductor 43 of the dipole antenna. Dipole antenna 40 includes atapered section 45 which serves as an impedance transformer between thecoaxial transmission line and antenna.

In operation, dipole antenna 40 receives RF energy from the RF generatorvia coaxial transmission line 20 and radiates the coal seam deposit inthe surrounding earth. As will be described in greater detail below, theradiated RF energy heats the coal and, in particular, vaporizes or boilsthe water in a specified region of the coal seam. By removing the waterfrom the coal seam, methane and other gases trapped within the coal seamare released and more easily removed.

Center conductor 22 of transmission line 20 is dual-purposed. The centerconductor not only serves as a part of the structure for heating thewater in the coal seam, it also provides an inner passage 42 forconveying the gas to the surface of the earth for processing. The gasenters inner passage 42 through intake 48. To remove the gas, a productreturn pipe 44 having a removable plug 46 extends from the end of centerconductor 22 at bellows 32.

RF gas recovery system 10 also includes a thermocouple assembly 50having a thermocouple coil 52 connected to bellows 32. Thermocouplecoils serve as a filter to “choke” or prevent the flow of low frequencycurrents to flow. Outer casing 14 also includes input pipes 56 throughwhich nitrogen gas is introduced within the casing. The nitrogen gas ismuch less flammable than oxygen and, therefore, provides a much saferenvironment for introducing high current levels from RF applicator 18.

The operation of this particular embodiment will now be described. Ingeneral, RF applicator 18 is placed within borehole 16 at a depth in arange between eight and 200 feet (e.g., 100 feet) at a locationapproximately central to a coalbed. RF energy at a power between 3 and20 KW (here, 10 KW), at a frequency of 27.12 megahertz (MHz) is providedto dipole antenna 40 from the RF generator. When the temperature at theapplicator well 20 reaches about 100 degrees C., the radiation power canbe cycled down to a lower power level sufficient for maintaining thetemperature until the temperature of the borehole 16 cools to apredetermined threshold (e.g., 90 degrees C.) and then the power iscyled back to 10 KW. The cycling of radiation power may be referred togenerally as modulating the power, or modulating the radation energy.Such modulation may also include cessation of the process.

It is also appreciated that the applicator well target temperaturesimplemented in the process may be slected to accommodate the temperaturetolerance of the components of RF oil recovery system 10 (e.g., a 150degree C. tolerance of the coaxial transmission line 20). It is alsoappreciated that the frequency of the radiated energy from the RFgenerator can be selected according to FCC regulations, and according toprinciples well known in the art, including the dielectric heatingcharacteristics of particular media. The energy may include radiofrequency energy and microwave energy. In this context, radio frequencyenergy has a frequency in the range between 300 kilohertz (KHz) and 300MHz, and microwave energy has a frequency in a range between 300 MHz and300 GHz.

The RF energy is transmitted from the RF generator to dipole antenna 40via coaxial transmission line 20. Dipole antenna 40 induces currentswithin the coal seam causing resistive and/or dielectric heating of thesurrounding region of the coal seam. The heating vaporizes or boils thewater in the coal seam creating a dry region. The dry region within thecoal seam is maintained by resistive hearing (low frequency) currents ordielectric (high frequency) currents and allows the trapped methane gasto be released. The released methane gas flows within outer casing 14 ofoil recovery system 10 and to inner passage 42 of center conductor 22via intake 48 where the methane gas is conveyed to the surface of theearth for processing. In particular applications, a gas filtering systemcan be positioned around RF applicator 14 (within or outside the borehole) to permit gas recovery through inner passage 42 without water. Thegas filtering system blocks liquid water and allows only the gas to passthrough it.

Other embodiments are within the scope of the claims. For example,although RF applicator 14 includes dipole antenna 40, other antennaconfigurations are equally applicable for use with the RF applicator.For example, referring to FIG. 3, RF applicator 14 can include anantenna 70 which is in the form of an extension of coaxial transmissionline 20.

The applicators described in conjunction with FIGS. 2 and 3 are designedto provide a predetermined impedance characteristic, for example, toprovide a high level of coupling into the coal seam. However, in otherembodiments, changing the impedance characteristics of the RF applicatormay be desirable. For example, dielectric characteristic of thesubterranean formation may differ or change as the water is converted tosteam. In such embodiments, the applicator may include a tuningmechanism.

Referring to FIG. 4, for example, a shorting link antenna 80 isconnected to the distal end of coaxial transmission line 20. In essence,shorting link antenna 80 is a dipole antenna having a looped end 82 andshorting link 84 positioned across the end. An insulated push rod 86 isconnected to shorting link 84 such that, in operation, it can be used tomove the shorting link and adjust the electrical length of the antenna.A remotely controlled, non-conducting hydraulic actuator 90 is providedto move push rod 86. In the embodiment shown, a center conductortransition 92 is provided between coaxial transmission line 20 and acenter conductor 94 of antenna 80. It is important to note that becauseantenna 80 has a looped end, center conductor 94 has a section offsetfrom the axis of coaxial transmission line 20.

In addition, collinear array antennas, such as those described in U.S.Pat. Nos. 4,583,589, 5,065,819, and 6,097,985, all of which areincorporated herein by reference, are also well-suited for use in RFapplicator 14. In addition, the “RF choke” structures described in thesereferences may be desirable for use to prevent the flow of certainfrequencies.

The applicators described above in conjunction with FIGS. 2–4 are oftenreferred to as electric antennas. Such antennas are well suited forapplications requiring a strong near electric field. In otherapplications, magnetically coupled antennas may be more suitable.Because the amplitude of the near field is relatively less than that ofan electrically coupled antenna, the risk of electric arcing is reduced,thereby increasing safety.

For example, referring to FIGS. 5 and 6, in still other embodiments,helical antennas 100 and 110 include multi-turn links surrounded by another helix. Specifically, FIGS. 5 and 6 show a twenty-turn link 102 andthree-turn link 112, respectively. Multi-turn links are multi-turn loopssurrounded by an outer helix 104 which, in turn, surrounds outerconductor 43 and is floating (i.e., has no ground plane). Outer helix104 is excited in the To mode by the multi-turn links. Excitation inthis manner is similar to exciting a rectangular waveguide in the TE₁₀mode with an electric monopole positioned along the centerline of abroad wall of the waveguide. Further details of antennas having thiscombination of elements can be found in U.S. Pat. No. 6,097,985.

Referring to FIG. 7, a helical antenna 130, similar to that of thehelical antenna 100 (shown in FIG. 5) includes a floating outer helix132, which unlike outer helix 104 of antenna 100 is positionedconcentrically within outer conductor 43.

Whether electrically coupled or magnetically coupled antennas, theapplicators are designed to maximize the impedance match between theapplicator and surrounding media.

Still other embodiments are within the scope of the claims.

1. A system for recovering gas trapped within the earth, the systemcomprising: a casing sized and configured to be positioned within aborehole in the earth, the casing formed of a material that istransmissive to electromagnetic energy and gas within the earth; a gasfiltering system positioned around the casing to permit gas to passthrough to the inside of the casing while blocking liquid from passingthrough to the inside of the casing; an antenna sized and configured tobe positioned within the casing, the antenna having a distal end and aproximal end and including: a radiating element at the distal end of theantenna which, in operation, transmits electromagnetic energy toward adesired area of the earth; and an interior channel for allowing gas tobe conveyed from the distal end to the proximal end of the antenna. 2.The system of claim 1, further comprising a product return pipe having afirst end connected to the proximal end of the antenna and a removablecap attached to a second end of the product return pipe.
 3. The systemof claim 1, further comprising a bellows connected to the proximal endof the antenna.
 4. The system of claim 1 further comprising athermocouple assembly connected to the proximal end of the antenna. 5.The system of claim 1 wherein the antenna is configured to operate in afrequency range between 300 KHz and 300 GHz.
 6. The system of claim 5wherein the antenna is configured to operate in a frequency rangebetween 1 MHz and 100 MHz.
 7. The system of claim 6 wherein the antennais configured to operate at a frequency of about 27 MHz.
 8. The systemof claim 6 wherein the antenna is configured to operate at a power levelin a range between 3 Kwatts and 20 Kwatts.
 9. The system of claim 8wherein the antenna is configured to operate at a power level of about10 Kwatts.
 10. A method for recovering gas trapped within the earth, themethod comprising: positioning a casing within a borehole in the earth,the casing formed of a material that is transmissive to electromagneticenergy and gas within the earth; positioning a gas filtering systemaround the casing to permit gas to pass through to the inside of thecasing while blocking liquid from passing through to the inside of thecasing; positioning an antenna within the casing, the antenna having adistal end and a proximal end, the antenna including: a radiatingelement at the distal end of the antenna which, in operation, transmitselectromagnetic energy toward a desired area of thy earth; and aninterior channel for allowing gas to be conveyed from the distal end tothe proximal end of the antenna; applying electromagnetic energy to theantenna to radiate the earth surrounding the is casing; drawing the gaswithin the earth into the interior channel of the antenna at the distalend of the antenna; and conveying the gas within the interior channel tothe proximal end of the antenna.
 11. The method of claim 10 furthercomprising attaching a first end of a product return pipe to theproximal end of the antenna and attaching a removable cap to a secondend of the product return pipe.
 12. The method of claim 10 furthercomprising attaching a bellows to the proximal end of the antenna. 13.The method of claim 10 further comprising attaching a thermocoupleassembly connected to the proximal end of the antenna.
 14. The method ofclaim 10 wherein the electromagnetic energy is in a frequency rangebetween 300 KHz and 300 GHz.
 15. The method of claim 14 wherein theelectromagnetic energy is in a frequency range between 1 MHz and 100MHz.
 16. The method of claim 15 wherein the electromagnetic energy has afrequency of about 27 MHz.
 17. The method of claim 15 wherein theelectromagnetic energy is at a power level in a range between 3 Kwattsand 20 Kwatts.
 18. The method of claim 17 wherein the electromagneticenergy is at a power level of about 10 Kwatts.
 19. A system forrecovering gas trapped within the earth, the system comprising: a casingsized and configured to be positioned within a borehole in the earth,the casing formed of a material that is transmissive to electromagneticenergy and gas within the earth; an antenna sized and configured to bepositioned within the casing, the antenna having a distal end and aproximal end and including: a radiating element at the distal end of theantenna which, in operation, transmits electromagnetic energy toward adesired area of the earth; and an interior channel for allowing gas tobe conveyed from the distal end to the proximal end of the antenna; anda bellows connected to the proximal end of the antenna.
 20. A system forrecovering gas trapped within the earth, the system comprising: a casingsized and configured to be positioned within a borehole in the earth,the casing formed of a material that is transmissive to electromagneticenergy and gas within the earth; an antenna sized and configured to bepositioned within the casing, the antenna having a distal end and aproximal end and including: a radiating element at the distal end of theantenna which, in operation, transmits electromagnetic energy toward adesired area of the earth; and an interior channel for allowing gas tobe conveyed from the distal end to the proximal end of the antenna; anda thermocouple assembly connected to the proximal end of the antenna.21. A method for recovering gas trapped within the earth, the methodcomprising: positioning a casing within a borehole in the earth, thecasing formed of a material that is transmissive to electromagneticenergy and gas within the earth; positioning an antenna within thecasing, the antenna having a distal end and a proximal end, the antennaincluding: a radiating element at the distal end of the antenna which,in operation, transmits electromagnetic energy toward a desired area ofthy earth; and an interior channel for allowing gas to be conveyed fromthe distal end to the proximal end of the antenna; attaching a bellowsto the proximal end of the antenna; applying electromagnetic energy tothe antenna to radiate the earth surrounding the is casing; drawing thegas within the earth into the interior channel of the antenna at thedistal end of the antenna; and conveying the gas within the interiorchannel to the proximal end of the antenna.
 22. A method for recoveringgas trapped within the earth, the method comprising: positioning acasing within a borehole in the earth, the casing formed of a materialthat is transmissive to electromagnetic energy and gas within the earth;positioning an antenna within the casing, the antenna having a distalend and a proximal end, the antenna including: a radiating element atthe distal end of the antenna which, in operation, transmitselectromagnetic energy toward a desired area of thy earth; and aninterior channel for allowing gas to be conveyed from the distal end tothe proximal end of the antenna; attaching a thermocouple assemblyconnected to the proximal end of the antenna; applying electromagneticenergy to the antenna to radiate the earth surrounding the is casing;drawing the gas within the earth into the interior channel of theantenna at the distal end of the antenna; and conveying the gas withinthe interior channel to the proximal end of the antenna.